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Journal ArticleDOI

Phosphogenesis and active phosphorite formation in sediments from the Arabian Sea oxygen minimum zone

15 Sep 2000-Marine Geology (Elsevier)-Vol. 169, Iss: 1, pp 1-20
TL;DR: In this paper, porewater chemistry, solid phase analysis and microscopic observations were combined to evaluate phosphogenesis in three boxcores located within the intensive oxygen minimum zone of the Arabian Sea.
About: This article is published in Marine Geology.The article was published on 2000-09-15 and is currently open access. It has received 127 citations till now. The article focuses on the topics: Francolite & Phosphorite.

Summary (4 min read)

1. Introduction

  • Phosphogenesis is the early diagenetic precipitation of francolite, a carbonate fluorapatite mineral (CFA).
  • The authors investigate phosphogenesis in three boxcores from the Arabian Sea located within oxygen-depleted bottom waters, one recovered from the sediments underlying the Oman upwelling system, and two from the Pakistan Margin.
  • In contrast to previous reports, the authors present evidence for Holocene phosphorite formation on the Oman Margin.

2.1. Sediment sampling and core description

  • All three boxcores are located within the OMZ and underlie an area of high primary productivity.
  • Bottom water oxygen (BWO) concentrations were obtained from nearby conductivity temperature depth (CTD) stations.
  • 14C accelerator mass spectrometry (AMS) dating was performed on handpicked non-coated foraminifers (Globorotalia menardii), coated foraminifers and phosphorite pellets.

2.2. Porewater analysis

  • Porewater extractions were started on board within 24 h of core collection according to shipboard routine (De Lange, 1992a).
  • The boxcores were vertically sluiced into a glovebox, which was kept under lowoxygen conditions (O2 , 0.0005%) and at in situ bottom water temperature.
  • Alkalinity was calculated after titration using the Gran plot method (Gieskes, 1973).
  • Porewater fluoride concentrations were measured with an ionspecific electrode.

2.3. Solid-phase analysis

  • The porosity and dry bulk density (DBD) were calculated from the weight loss after drying at 608C, assuming a sediment density of 2.65 g cm23.
  • Relative errors for duplicate measurement were better than 3%, except for Zr and Ti (5%).
  • A separate sequential extraction consisting of eight times the 2 M NH4Cl step was performed for some sediment samples, where each extracted solution was analysed separately.
  • All extracted solutions were measured for P with ICP-AES.

2.4. Description of the model

  • A diagenetic model for P cycling developed by Slomp et al. (1996) was applied to the porewater and sequential extraction results for BC451 and BC455.
  • This steady state model describes the concentration change with depth of porewater phosphate and three forms of solid-phase P, namely organic P, Febound P and authigenic P.
  • The processes (1)–(4) are described as first-order reactions, with reaction rate constants kg, ks, km and ka, respectively.
  • Values ofkg, km, ka, JAx 0 and JGx 0 were varied to fit the model to the experimental data.
  • Extra weight was assigned to the data points in the upper part of each profile and the whole authigenic P profile.

3.1. Porewater

  • In BC451 and BC455, the phosphate concentrations are lower, and the decrease of phosphate with depth is less pronounced (BC451) or absent (BC455).
  • Fluoride concentrations decrease with depth in all three boxcores, whereas the ammonium concentrations and alkalinity increase almost linearly with depth (Fig. 2).
  • BWO concentrations for all three boxcores are below the detection limit (2 mM; Table 1).

3.2. Solid phase

  • The P concentration in this core gradually decreases with depth to ca. 4000 ppm.
  • Approximately 20% of the solid-phase P is present in the fraction smaller than 65mm, which constitutes 50 wt% of the bulk sediment.
  • In BC451 and BC455, the P fraction responsible for the increase with depth of total solid-phase P was extracted during steps 1 and 2 (Fig. 3).
  • The Corg/Ntot weight ratio in TC484 increases with depth till 18 cm, where it reaches a constant value of 10 (Fig. 4).
  • The fish debris concentration (numbers per gram of the 150–595mM sieve fraction) decreases with depth in BC484 (Fig. 5), and correlates reasonably well with the total P concentration.

3.3. Microscopic observations and calibrated14C ages of apatite macro particles in BC484

  • Microscopic observations and microprobe analysis allowed the identification of three types of apatite macro particles in BC484: coated foraminifers, phosphatised pellets and fish debris.
  • In the deeper part of the boxcore, coated foraminifers become less frequent.
  • Calibrated14C ages for coated foraminifers are higher than “clean” foraminifers in the same sediment interval (Table 3).
  • Their surface is blackish/brownish and usually smooth.
  • Thin slides of samples reveal no internal structures indicating that the pellets are probably composed of apatite micro crystals.

3.4. Application of the model

  • The porewater equilibrium concentration for francolite formation (Ca) may vary between 0.4 and 11mM for pH 4 (Atlas and Pytkowicz, 1977).
  • Here, a Ca concentration of 10mM was used for both cores, which equals the porewater phosphate concentration at greater sediment depth (Schenau, 1999).
  • The deposition rate of Fe-bound P (JMx 0) was estimated from the mass accumulation rate, an average reactive iron concentration of 6000 ppm (equal to the concentration in surface sediments below the OMZ) and an atomic Fe/P ratio of 20 for the newly deposited reducible iron particles (Schenau, 1999).
  • The model fits agree reasonably well with the measured data (Fig. 6), with the exception of the porewater profiles of BC451 and BC455.

4.1. Authigenic apatite formation

  • Three indicators have been studied to examine whether phosphogenesis is currently taking place in the sediments located within the OMZ of the Arabian Sea (Ruttenberg and Berner, 1993): (1) porewater phosphate and fluoride concentrations; (2) the saturation state of francolite; and (3) solid-phase authigenic P concentrations.
  • A decrease in porewater phosphate and fluoride concentration with depth is indicative for P and F removal to the solid phase (Jahnke et al., 1983; Ruttenberg and Berner, 1993).
  • The number of fish debris, as counted in the 150–595mm sieve fraction, however, does not clearly increase with depth (Fig. 5).
  • Therefore, the authors argue that the increase of the PNH4Cl fraction with depth in BC451 and BC455 is the result of precipitation of an authigenic Ca-phosphate mineral, which is more soluble than francolite.
  • Laboratory experiments have shown that francolite precipitation at high phosphate concentrations is a two-step process.

4.2. Implications of the model

  • The model results confirm that the increase of the solid-phase P with depth at stations BC451 and BC455 can be explained by early diagenetic phosphogenesis.
  • The calculated francolite formation rates correspond reasonably well with the downwardJP(F) fluxes (Table 6).
  • As a result, the model predicts that all degradable organic P is mineralised in the upper few centimetres of the sediment (Fig. 6).
  • This may also explain the discrepancy between the observed and the modelled phosphate porewater profiles.
  • Early diagenetic iron redox cycling has been shown to be important for phosphogenetic processes in certain marine environments (e.g.

4.3. Phosphorite formation in BC484

  • Many of these phosphorite deposits have been identified as lag deposits (Kolodny, 1981; Garrison and Kastner, 1990).
  • Beside redeposition processes, winnowing has been suggested to play an important role in the formation of phosphorites (e.g. Glenn and Arthur, 1988; Glenn et al., 1994).
  • Since phosphorite particles have a higher specific gravity than the surrounding detrital particles, bottom currents could wash away the finer, lighter particles and thus concentrate P in the top of the sediment.
  • Winnowing causes low sedimentation rates, which in turn may enhance the growth of phosphorite nodules by keeping them in the zone of active diagenesis.

4.3.1. Winnowing

  • The high phosphorus concentration in the top 20 cm of BC484 could be the result of winnowing, which is an important process affecting the continental slope sediments on the Oman Margin (Shimmield et al., 1990; Pedersen et al., 1992).
  • A comparison between BC484 and TC484 of the Corg and Ptot profiles revealed an offset of 10 cm.
  • These low Corg/Ntot ratios are common for many recent sediments that have not been subject to winnowing (e.g. Calvert et al., 1995; Van der Weijden et al., 1999).
  • O’Brien et al. (1990) found a close correlation between bottom water current velocity and the CaCO3 content of sediments on the East Australian continental margin.
  • The Ca concentration in the phosphorus-rich layer is in fact lower than deeper in the sediment, indicating that the top is not winnowed.

4.3.2. Redeposition

  • The continental slope of the Oman Margin is particularly steep (Prell and shipboard party of ODP Leg 117, 1990) and, therefore, redeposition processes are likely to occur.
  • The calibrated14C ages for “clean” (i.e. un-coated) foraminifers indicate normal sedimentation for the last 13,000 yr (Table 3).
  • The authors argue that the phosphatised material from in BC484 originates from two different sources.
  • The downward flux of fluoride may thus account for the high solid-phase P content in the top of BC484.
  • As a consequence, recent phosphorite formation on the Oman Margin may have remained unobserved thus far.

5. Conclusions

  • Porewater and solid-phase P speciation results indicate that phosphogenesis is occurring in the surface sediments located within the OMZ of the Arabian Sea.
  • The precipitation of a precursor precedes francolite formation in the sediments on the Karachi Margin.
  • Early diagenetic iron cycling does not significantly affect sedimentary P cycling in these environments.
  • This implies that dysoxic rather than fully anoxic bottom waters may be more effective in promoting early diagenetic phosphogenesis.

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Citations
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Journal ArticleDOI
01 Apr 2008-Elements
TL;DR: In this article, the cycle of the Earth's phosphorus cycle has been studied and the effect of human activities on the cycling of the phosphorus cycle is discussed, which has led to increased use of fertilizers, deforestation and soil loss, and sewage sources.
Abstract: The cycling of phosphorus, a biocritical element in short supply in nature, is an important Earth system process. Variations in the phosphorus cycle have occurred in the past. For example, the rapid uplift of the Himalayan-Tibet Plateau increased chemical weathering, which led to enhanced input of phosphorus to the oceans. This drove the late Miocene “biogenic bloom.” Additionally, phosphorus is redistributed on glacial timescales, resulting from the loss of the substantial continental margin sink for reactive P during glacial sea-level lowstands. The modern terrestrial phosphorus cycle is dominated by agriculture and human activity. The natural riverine load of phosphorus has doubled due to increased use of fertilizers, deforestation and soil loss, and sewage sources. This has led to eutrophication of lakes and coastal areas, and will continue to have an impact for several thousand years based on forward modeling of human activities.

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Book ChapterDOI
01 Jan 2007
TL;DR: In this article, the elemental proxies for palaeoclimatic and palaeoceanographic variability in marine sediments are discussed, and the application of sedimentary geochemistry to the reconstruction of climatic and oceanographic changes over the Cenozoic is discussed.
Abstract: Publisher Summary This chapter discusses the elemental proxies for palaeoclimatic and palaeoceanographic variability in marine sediments. Physical and biological processes during deposition coupled with post-depositional chemical reactions yield a complex component mixture that can provide significant palaeoceanographic and palaeoclimatic information to complement and strengthen interpretations derived from the study of microfossils and the isotopic compositions of sedimentary components. The chapter reviews the application of sedimentary geochemistry to the reconstruction of climatic and oceanographic changes over the Cenozoic, with emphasis on the Late Pleistocene. Records from both pelagic regimes and ocean margin provinces are used to show how information on both ocean conditions and terrestrial climates can be assembled from the major, minor and trace element composition of sea-floor deposits. The chapter provides background sedimentary information, and describes some common data manipulation techniques that are used to construct and study palaeoclimatic and palaeoceanographic records in marine sediments.

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Journal ArticleDOI
21 Jan 2005-Science
TL;DR: It is shown that apatite abundance in sediments on the Namibian shelf correlates with the abundance and activity of the giant sulfur bacterium Thiomargarita namibiensis, which suggests that sulfur bacteria drive phosphogenesis.
Abstract: Phosphorite deposits in marine sediments are a long-term sink for an essential nutrient, phosphorus. Here we show that apatite abundance in sediments on the Namibian shelf correlates with the abundance and activity of the giant sulfur bacterium Thiomargarita namibiensis, which suggests that sulfur bacteria drive phosphogenesis. Sediments populated by Thiomargarita showed sharp peaks of pore water phosphate ( /=50 grams of phosphorus per kilogram). Laboratory experiments revealed that under anoxic conditions, Thiomargarita released enough phosphate to account for the precipitation of hydroxyapatite observed in the environment.

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TL;DR: In this paper, the authors present an overview of the natural (pre-human) and modern (synhuman) global P mass balances, followed by in-depth examinations of several current areas of research in P cycling, including climatic controls on ecosystem dynamics and soil development, the control of oxygen on coupled P and Carbon cycling in continental margins, and the role that P plays in controlling ocean productivity on Cenozoic timescales.
Abstract: Phosphorus (P) is a limiting nutrient for terrestrial biological productivity that commonly plays a key role in net carbon uptake in terrestrial ecosystems (Tiessen et al. 1984, Roberts et al. 1985, Lajtha and Schlesinger 1988). Unlike nitrogen (another limiting nutrient but one with an abundant atmospheric pool), the availability of “new” P in ecosystems is restricted by the rate of release of this element during soil weathering. Because of the limitations of P availability, P is generally recycled to various extents in ecosystems depending on climate, soil type, and ecosystem level. The release of P from apatite dissolution is a key control on ecosystem productivity (Cole et al. 1977, Tiessen et al. 1984, Roberts et al. 1985, Crews et al. 1995, Vitousek et al. 1997, Schlesinger et al. 1998), which in turn is critical to terrestrial carbon balances (e.g., Kump and Alley 1994, Adams 1995). Furthermore, the weathering of P from the terrestrial system and transport by rivers is the only appreciable source of P to the oceans. On longer time scales, this supply of P also limits the total amount of primary production in the ocean (Holland 1978, Broecker 1982, Smith 1984, Filippelli and Delaney 1994). Thus, understanding the controls on P weathering from land and transport to the ocean is important for models of global change. In this paper, I will present an overview of the natural (pre-human) and modern (syn-human) global P mass balances, followed by in-depth examinations of several current areas of research in P cycling, including climatic controls on ecosystem dynamics and soil development, the control of oxygen on coupled P and Carbon (C) cycling in continental margins, and the role that P plays in controlling ocean productivity on Cenozoic timescales. ### Natural (pre-human) phosphorus cycle The human impact …

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Cites background from "Phosphogenesis and active phosphori..."

  • ...Several recent studies suggest that P may exhibit a significant preferential regeneration compared to C during diagenesis of organic matter in low oxygen continental-margin sediments (Ingall et al. 1993, Compton et al. 1993, Ingall and Jahnke 1994, Schenau et al. 2000)....

    [...]

Book ChapterDOI
TL;DR: A brief overview of the various components of the global phosphorus cycle is given in this paper, including a discussion of the most pressing research questions currently being posed and research efforts presently underway to address these questions.
Abstract: Phosphorus is an essential nutrient for all life-forms It is a key player in fundamental biochemical reactions involving genetic material (DNA and RNA) and energy transfer (ATP) and in structural support of organisms provided by membranes (phospholipids) and bone (the biomineral hydroxyapatite) Photosynthetic organisms utilize dissolved phosphorus, carbon, and other essential nutrients to build their tissues using energy from the sun Biological productivity is contingent upon the availability of phosphorus to these simple organisms that constitute the base of the food web in both terrestrial and aquatic systems It begins with a brief overview of the various components of the global phosphorus cycle Estimates of the mass of important phosphorus reservoirs, transport rates (fluxes) between reservoirs Following the overview, various aspects of the global phosphorus cycle are examined in more depth, including a discussion of the most pressing research questions currently being posed and research efforts presently underway to address these questions

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References
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Journal ArticleDOI
TL;DR: In this article, the growth of fluorapatite (FAP) in carbonate-free NaCl-CaCl2-NaF-Na2HPO4 solutions, at seawater calcium concentration, chlorinity, and pH.

145 citations


"Phosphogenesis and active phosphori..." refers background in this paper

  • ...First, an amorphous, F-poor precursor is formed, which subsequently acts as a substrate for francolite precipitation ( Van Cappellen and Berner, 1991; Krajewski et al., 1994)....

    [...]

Journal ArticleDOI
TL;DR: This paper showed that dissolution of phosphatic fish debris represents a mechanism for remineralization of phosphate comparable to or larger in magnitude than that by oxidative regeneration of organically bound P. This mechanism may also control widespread phosphorite formation in this area.

143 citations


"Phosphogenesis and active phosphori..." refers background in this paper

  • ...Sundby et al., 1992; Slomp et al., 1998) and dissolution of fish debris ( Suess, 1981 )....

    [...]

Journal ArticleDOI
TL;DR: In this paper, the authors examined bioturbation processes at six stations (400, 700, 850, 1000, 1250 and 3400m) along a transect across the oxygen minimum zone (OMZ) on the Oman margin.
Abstract: Oxygen minimum zones are expected to alter substantially the nature, rates and depths of bioturbation along continental margins, yet these effects remain poorly studied. Using excess 210 Pb profiles, sediment X-radiography and box-core samples for macrofauna, we examined bioturbation processes at six stations (400, 700, 850, 1000, 1250 and 3400 m deep) along a transect across the oxygen minimum zone (OMZ) on the Oman margin. Bottom-water oxygen concentrations ranged from ∼0.13 ml l−1 at 400 m to ∼2.99 ml l−1 at 3400 m. 210 Pb mixed-layer depth and bioturbation intensity (Db) exhibited high within-station variance, and means did not differ significantly among stations. However, the mean mixed-layer depth (4.6 cm) for pooled OMZ stations (400–1000 m depths, 0.13–0.27 ml l−1 bottom-water oxygen) was half that for stations from similar water depths along well-oxygenated Atlantic and Pacific slopes (11.1 cm), suggesting that oxygen stress reduced 210 Pb mixing depth on the Oman margin. Modal burrow diameter and the diversity of burrow types at a station were highly correlated with bottom-water oxygen concentration from the edge to the core of the Oman OMZ (Spearman's rho⩾0.89, p⩽0.02), suggesting that these parameters are useful proxies for bottom-water oxygen concentrations under dysaerobic conditions. In contrast, neither the maximum diameter and nor the maximum penetration depth of open burrows exhibited oxygen-related patterns along the transect. Reduced 210 Pb mixing depth within the Oman-margin OMZ appeared to result from a predominance of surface-deposit feeders and tube builders within this zone, rather than from simple changes in horizontal or vertical distributions of macrofaunal abundance or biomass. The number of burrow types per station was highly correlated with macrofaunal species diversity, suggesting that burrow diversity may be a good proxy for species diversity in paleo-dysaerobic assemblages. We conclude that bottom-water oxygen concentrations of 0.13–0.27 ml l−1 substantially alter a number of bioturbation parameters of importance to diagenetic and biofacies models for continental margins.

115 citations


Additional excerpts

  • ...Sea slope sediments located within the OMZ ( Smith et al., 2000 ), which are similar to the values used in the model (Table 4). Extensive bioturbation enhances phosphate release to the water column, reduces the accumulation of dissolved phosphate and, accordingly, inhibits phosphogenesis (Filippelli and Delaney, 1994)....

    [...]

Book ChapterDOI
01 Jan 1992
TL;DR: The basic controls on ocean productivity are poorly understood both biologically and geologically as mentioned in this paper, and we do not know the global patterns of productivity very well, either with regard to the rates of primary production (that is, the amount of carbon fixed in the photic zone each year).
Abstract: The basic controls on ocean productivity are poorly understood both biologically and geologically. In fact, we do not know the global patterns of productivity very well, either with regard to the rates of primary production (that is, the amount of carbon fixed in the photic zone each year), or with regard to the types of primary production (that is, the kinds of organisms involved).

110 citations

Frequently Asked Questions (1)
Q1. What contributions have the authors mentioned in the paper "Pii: s0025-3227(00)00083-9" ?

In this study, porewater chemistry, solid-phase analysis and microscopic observations were combined to evaluate phosphogenesis in three boxcores located within the intensive oxygen minimum zone of the Arabian Sea. Authigenic apatite precipitation rates vary between 0. 076 and 1. 04 mmolP cm yr, and are of the same order of magnitude as reported for other high productivity areas. This observation contrasts with previous reports of only old phosphorites in this area. Model results suggest that sediment mixing is essential in promoting early diagenetic phosphogenesis.